The present invention relates to a spinel-type lithium manganese complex oxide for use as a cathode active material of a lithium secondary battery and a process for producing it. The spinel-type lithium manganese complex oxide is useful as a cathode active material of, for instance, a 4-V lithium secondary battery.
As a method of producing a spinel-type lithium manganese complex oxide which is used as a cathode active material of a lithium secondary battery, the following methods have been proposed so far.
(a) A powder method in which lithium carbonate and manganese dioxide powders are mixed with each other, and the mixture is calcined at approximately 800xc2x0 C.
(b) A melt-impregnation method. Lithium nitrate or lithium hydroxide which is easy to melt below 500xc2x0 C. is penetrated into porous manganese dioxide, and calcined.
(c) A method in which lithium nitrate and manganese nitrate are dissolved in water, and the solution is ultrasonically spray-pyrolyzed.
As a lithium manganese complex oxide which is appropriate as a cathode active material of a lithium secondary battery, the following complex oxides have been proposed.
(d) LiMn2O4 (Japanese Patent Publication No. 21,431/1996)
(e) LixMn2O4 in which x is 0.9xe2x89xa6xxe2x89xa61.1 excluding x=1.0 (Japanese Patent Publication No. 21,382/1996)
(f) Li2Mn4O9, Li4Mn5O12 [J. Electrochem Soc., vol. 139, No. 2, pp. 363-366 (1992)]
(g) LixMn2Oy in which x and y are 1.0 less than x less than 1.6, 4.0 less than y less than 4.8, and 8/3+(4/3) x less than y less than 4.0+(1/2)x [Japanese Laid-Open (Kokai) No. 2,921/1996]
(h) Li1+xMn2xe2x88x92xO4 in which x is 0, 0.03, 0.05, 0.10, 0.22, 0.29 or 0.33 [Solid State Ionics 69 (1994), pp. 59-67]
The above-mentioned methods have, however, involved the following problems.
In the powder method (a), carbonate and oxide powders are used as starting materials and it is, therefore, necessary to be calcined the same at a relatively high temperature. Accordingly, a defective spinel such as an excess oxygen spinel tends to be formed. Further, it is impossible to uniformly mix the powders at a molecular level. For example, not only intended LiMn2O4 but also Li2MnO3 and LiMnO2 are sometimes formed. In order to prevent formation of such complex oxides, calcination for a long period of time has to be repeated several times while adjusting the content of oxygen.
The melt-impregnation method (b) improves the uniformity of Li and Mn in comparison with the solid phase method. However, a porous manganese material is required as a starting material. In order to obtain this porous manganese material, a milling treatment is needed. For this milling treatment, a special mill has to be used, and impurities from grinding media and mill lining are inevitable. These impurities decrease the qualities of the resulting complex oxide powder as a cathode active material and increase the cost. Further, unless the calcination is conducted for a long period of time at a low temperature to avoid vaporization of the lithium starting material whose melting point is low, the crystallinity of the resulting complex oxide is decreased. Accordingly, when the complex oxide is used as an active material of a secondary battery, the crystal structure collapses during repetition of the charge-discharge cycle of the battery, decreasing the capacity of the secondary battery. Still further, when Mn is substituted by a cation having a low valence and an ionic radius close to that of Mn, such as Fe, Co, Ni or Mg, to improve the high-rate discharge or the charge-discharge cycle characteristics of the secondary battery, it is unescapable that the distribution of Mn and the substituent cation is non-uniform in this melt-impregnation method.
In the spray pyrolysis method (c), the salts constituting the spinel-type lithium manganese complex oxide can uniformly be mixed at the ionic level to outstandingly increase the uniformity as compared to the melt-impregnation method. Further, since the step of milling the starting materials is not needed, impurities formed during the milling step can be prevented. However, in this spray pyrolysis method, the period of thermal treatment is too short because a series of steps of evaporation of the solvent and thermal-decomposition are conducted within a few seconds. The conventional calcination treatment and the crystallinity of the resulting complex oxide is not good. Accordingly, when the complex oxide is used as an active material of a secondary battery, the crystal structure collapses during repetition of the charge-discharge cycle of the battery to decrease the capacity of the secondary battery. Further, since the specific surface area of the resulting complex oxide is as large as tens of square meters per gram, an electrolyte in contact with this complex oxide becomes decomposed, sometimes notably decreasing the charge-discharge cycle characteristics and the storage properties of the secondary battery.
Still further, the compositions of the above-mentioned lithium manganese complex oxides are problematic in the following points.
When the composition (d) is used as a cathode active material of a secondary battery, the capacity of the battery is decreased to 50% of the original capacity in a matter of tens of charge-discharge cycles.
When the composition (e) is used as a cathode active material of a secondary battery and x is 0.9xe2x89xa6xxe2x89xa61.0, the amount of the lithium ion taken out by the initial charge decreases causing a decrease of the capacity of the battery. When x is 1.0 less than xxe2x89xa61.1, the crystal structure is changed from a cubic system of a 4-V region to a tetragonal system of a 3-V region through Jahn-Teller phase transition causing decrease of the capacity of the battery in the repetition of the charge-discharge cycle.
The composition (f) has an operating potential in the approximately 3.0 V region, and therefore cannot be used as a cathode active material of a 4-V region lithium battery.
The composition (g) includes a composition close to the composition (f) of the 3-V region. In order to form these active materials, a manganese starting material having a specific surface area of from 5 to 50 m2/g is needed. However, a powder having a large specific surface area has a strong cohesive force and cannot uniformly be mixed with the lithium starting material. It is further necessary that after the manganese and lithium starting materials are mixed, the mixture is calcined at 500xc2x0 C. or less for 2 hours or more and then at 850xc2x0 C. or less for from 1 to 50 hours. As a result, the productivity is poor.
In the composition (h), a part of a manganese site is substituted with lithium to control the Jahn-Teller phase transition and improve the cycle characteristics. Nevertheless, the mere substitution of manganese with 3.0% lithium decreases the discharge capacity of approximately 20%.
Accordingly, it is an object of the present invention to solve the above-mentioned problems, and provide a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery and a process for producing it. The spinel-type lithium manganese complex oxide can be used as a cathode active material of a 4-V region lithium secondary battery having a large charge-discharge capacity and exhibiting excellent charge-discharge cycle characteristics.
The present invention provides a spinel-type lithium manganese complex oxide for a cathode active material of a lithium secondary battery, which is characterized in that said spinel-type lithium manganese complex oxide has an average particle diameter between about 1 and 5 micrometers, and a specific surface area between about 2 and 10 m2/g.
In the above spinel-type lithium manganese complex oxide, said spinel-type lithium manganese complex oxide may be represented by the formula Li(Mn2xe2x88x92xLix) O4 wherein x is 0xe2x89xa6xxe2x89xa60.1 and more preferably, x is 0 less than x less than 0.02. In the above spinel-type lithium manganese complex oxide, preferably Mn is partially substituted by Cr, Ni, Fe, Co, Mg or Li.
This described composite complex oxide solves the above mentioned problem having a high surface activity which is appropriate as a cathode active material of a lithium secondary battery.
The present invention further provides a process for producing the spinel-type lithium manganese complex oxide comprises the steps of: 1) atomizing and pyrolyzing an aqueous solution and/or an alcohol solution of compounds containing metallic salts constituting a spinel-type lithium manganese complex oxide to obtain said complex oxide, and 2) annealing said spinel-type lithium manganese complex oxide to increase the average particle diameter thereof to about between about 1 and 5 micrometers and adjust the specific surface area thereof to between about 2 and 10 m2/g.
In the above process, the atomizing and pyrolyzing temperature may be between about 500 and 900xc2x0 C., and the annealing temperature may be between about 600 and 850xc2x0 C.
In the above process, the metallic salts may be at least one of lithium nitrate, lithium acetate and lithium formate and at least one of manganese nitrate, manganese acetate and manganese formate.
When the aqueous solution and/or the alcohol solution containing the metallic salts constituting the spinel-type lithium manganese complex oxide is atomized into a heated atmosphere, heat decomposition occurs instantaneously to cause fine droplets due to a self-chemical decomposition. Consequently, a fine complex oxide having the high surface activity can be formed. When this complex oxide is then annealed, the average particle diameter is increased to between about 1 and 5 micrometers, and the specific surface area is adjusted to between about 2 and 10 m2/g. Thus, the composite complex with the high surface activity which is appropriate as a cathode active material of a lithium secondary battery can be obtained.
The metallic salts constituting the spinel-type lithium manganese complex oxide are of Li and Mn as well as substituents for improving charge/discharge characteristics (such as Cr, Ni, Fe, Co, Mg and Li) added as required for substitution of the Mn site. In this connection, Li as a substituent of the Mn site (octahedral site) is different from Li in the tetrahedral site. Typical examples of the water-soluble compounds comprising these metal salts include acetate, formate, nitrate and chloride. These compounds are much less costly than an organic complex in which a hydrogen ion in the molecule is substituted with a metallic ion, such as an alkoxide. With these compounds, the cost of starting materials can be reduced, and this is industrially advantageous.
Thus, the process of the present invention can provide a uniform spinel-type lithium manganese complex oxide having an average particle diameter of between about 1 and 5 xcexcm and a specific surface area of between about 2 and 10 m2/g.
Accordingly, when this complex oxide, preferably the complex oxide represented by the formula Li(Mn2xe2x88x92xLix) 04 in which x is 0xe2x89xa6xxe2x89xa60.1, more preferably the complex oxide in which x is 0 less than x less than 0.02, is used as a cathode active material of a secondary battery, a lithium secondary battery which is excellent in the charge-discharge cycle characteristics and the storage characteristics can be obtained.
The present invention is illustrated specifically by referring to the following embodiments.